docs: def _sigma_ee_nonrel(self, gam, eps):
          """
          Eq. A5 of Baring et al. (1999)
          Use for Ee < 2 MeV
          """
          s0 = 4 * r0 ** 2 * alpha / (15 * eps)
          x = 4 * eps / (gam ** 2 - 1)
          sigma_nonrel = s0 * self._F(x, gam)
          sigma_nonrel[np.where(eps >= 0.25 * (gam ** 2 - 1.0))] = 0.0
          sigma_nonrel[np.where(gam * np.ones_like(eps) < 1.0)] = 0.0
          return sigma_nonrel / mec2_unit